Graphic recording apparatus and method

ABSTRACT

The disclosure describes apparatus for preventing scalloping the photographic film image produced by a scintillation scanner. The described scanner comprises a detector which can be scanned along parallel paths over an area of interest in a patient. The detector is directly coupled to a light-emitting tube that simultaneously is scanned in a similar manner over a photographic film. Gamma rays emitted by the patient are received by the detector and are converted into corresponding electrical event pulses and bits of event information which are sequentially shifted through shift registers in response to periodic clock pulses. A stepping motor moves the detector and light-producing tube a predetermined distance in response to the receipt of each clock pulse so that the concentration of gamma ray events occuring along any particular segment of the detector scan path can be precisely determined. An individual event pulse is transmitted to the light-producing tube after is has been shifted halfway through the shift register, so that the information stored in the shift register represents gamma ray events occurring before and after the gamma ray event resulting in the event pulse. A counter continuously analyzes the information flowing into and out of the shift register so that an accurate count of the information bits stored in the shift register is continuously available. Additional means are provided for controlling the intensity of the light produced by the tube in response to the value of the information stored in the shift register. As a result, the light intensity produced in response to any single gamma ray event depends on the concentration of gamma ray events occurring on either side of the single event along the detector scan path.

[ Oct. 23, 1973 United States Patent 1 McMillian et al.

GRAPHIC RECORDING APPARATUS AND METHOD [75] Inventors: Lonnie S.McMillian; Arthur H.

scalloping the photographic film image produced by a scintillationscanner. The described s a detector which can be scanned al over an areaof interest in a canner comprises :Ti Gard of directly coupled to alight-emittin [73] Assignee: Abbott/Laboratories, North mk a r. mm u cmkm e mdm h We... h

e msTb. OLm m n. a P

neously is scanned in a similar manner over a photographic film. Gammarays emitted b e. .18 to wc n peld hm r w Chicago, Ill. received by thedetector and are con s ondin electrical event ulses and bits of eventin- [22] Filed: 1972 f rmatio n which are seque ntially shifted throughshift registers in response to periodic clock pulses. ping motor movesthe detector and 1 tube a predetermined distance in res Astepight-producing ponse to the receipt of each clock pulse so that theconcentration of gamma ray events occuring along any particular segmentof the detector scan path can be precisely determined. An individualevent pulse is transmitted to the light-producing tube after is has beenshifted halfway through the shift register, so that the informatio S4 4sm 1 mam WORM 2H6 a S 3 m m 7 n n J m u n a 2 In J 8 0 ans N C M. M Pmfw A UIF .11] l 2 8 2 555 [56] References Cited UNITED STATES PATENTS ygamma ray stored in the shift register represents gamma ra eventsoccurring before and after the event resulting in the event pulse.

Hansen l78/6.7 R

Primary Examiner-James W. Moffit Att0rneyMolmare, Allegrettr, Newitt &Wrtcoff 21 Claims, 14 Drawing Figures [57] ABSTRACT I The disclosuredescribes apparatus for preventing PAIENTEUumeamn 3 sum 110$ 11 767.850

GRAPHIC RECORDING APPARATUS AND METHOD BACKGROUND OF THE INVENTION Thisinvention relates to scintillation scanners, and more particularlyrelates to apparatus for controlling the radiographic image produced byascintillation scanner.

Scintillation scanners are well known to the medical profession and areuseful for diagnosing and detecting the location of diseased tissue,such as tumors. In order to use a scintillation scanner for thispurpose, a patient is given a special substance that is selectivelyabsorbed by a particular tissue of the body, such as the thyroid, lungs,brain, or liver. When the substance is manufac tured, it is doped with aradioisotope which radiates gamma rays. By measuring the concentrationof gamma rays emitted from a patient, the extent to which the substancehas been absorbed by the tissue can be determined by means of ascintillation scanner.

A scintillation scanner for detecting gamma rays emitted from a patientgenerally consists of a lead collimator which passes only gamma raysthat are emitted from a very small volume within the tissue. A specialgamma-sensitive crystal is used to convert these gamma rays passed bythe collimator into small light flashes which are amplified andconverted into electrical pulses by a photomultiplier tube. Theelectrical pulses are generally passed through a pulse height analyzerwhich eliminates the effect of gamma rays that are outside the knownenergy spectrum of the gamma rays emitted by the radioisotope in thesubstance.

The crystal and associated collimator are moved across the patient bymeans of a mechanical scanning arrangement so that the tissue ofinterest in the patient is covered by a rectilinear scanning patterncarefully controlled as to speed and location. The mechanical scanningarrangement simultaneously moves a light source over a photographic filmwith the same scanning motion. For every gamma ray of the proper energyspectrum received by the collimator, a pulse of light is generated bythe light source and is used to expose the film. The film is laterdeveloped so that a synthesized image of the tissue can be observed andused for diagnostic purposes.

A variety of control devices have been used in the past in order tocontrol the manner in which light is generated by the light source. Forexample, it has been.

found that the image on the film resulting from a scintillation scannercan be improved for diagnostic purposes by a technique known asbackground erase. According to this technique, gamma rays originatingfrom an area of relatively high gamma ray concentration produce an imageon the film, whereas gamma rays originating from an area of relativelylow gamma ray concentration produce no image on the film. This techniqueis particularly useful when the radioisotopedoped substance is trappedin the tissue of interest in high concentration, and is trapped insurrounding tissue in relatively low concentration. In this situation,the tissue of interest produces an easily-visible image on the film,whereas the surrounding tissue produces no image at all.

One method of implementing a background erase mode of operation is todetermine an average activity level, or gamma ray count rate persquarecentimeter, and to use some percentage of the average as athreshold level below which pulses are rejected, and above which pulsesare passed to expose the film. This method of implementation generallyhas the effect of passing all pulses when the average gamma ray activityrate is high (such as in a tissue of interest) and in rejecting allpulses when the average gamma ray activity rate is low (such as in areassurrounding the tissue of interest). Normally, the average activitylevel is determined by counting pulses per unit time with an electroniccounter which is periodically reset by an arbitrary clock source that isrelated to real time. Since the clock source is not synchronized withthe motion of the gamma ray detector, the averaging of pulses per unittime generally does not occur over the same relative scan area ofadjoining scan paths. For example, if the scintillation scanner is inmotion at the border line of an area of low activity and an area of highactivity, the activity level might be averaged primarily in the area ofhigh activity during one scan and might be averaged primarily in thearea of low activity on the next scan. This mode of operation results inan asymmetrical appearance of the resulting photographic image whichcomplicates diagnosis.

Accordingly, one object of the present invention is to relate theaveraging of gamma ray activity to an actual segment of a detector scanpath so as to produce a consistent image from one scan path to the next.

Previous methods of implementing a background erase mode of operationare also deficient in that they create so called scalloping" of theresulting photographic image. Scalloping" occurs primarily because thedecision whether to record a particular gamma ray event is based on theaverage activity level of events occurring in the past. For example, ifthe gamma ray detector is in an area of low activity moving toward anarea of high activity, no light exposes the film since the averageactivity level is below the selected threshold level. As soon as theboundary of the high activity area is reached, the average activitylevel begins to increase. However, the high activity area must bepenetrated for some time before the average activity level risessufficiently to exceed the selected threshold level. As a result, lightpulses appear on the film past the boundary of the high activity area,rather than at the boundary.

The foregoing mode of operation is reversed when the scanner istraveling from an area of high activity to an area of low activity. Inthis case, the average activity level remains high for some time afterthe boundary is passed, so that light pulses appear on the film beyondthe boundary line into the area of low activity. As a result, lightpulses are indented in one direction along one scan path and areindented in the opposite'direction along an adjoining scan path, so thata scalloping effect is visible on the film.

Accordingly, it is another object of the present invention to provideimproved techniques to eliminate scalloping" and to produce aphotographic image which accurately records the gamma ray activity ofthe area scanned.

Experience has also shown that a photographic image resulting from ascintillation scanner can be more easily interpreted if a mode ofoperation known as contrast enhancement is employed. One method ofimplementing contrast enhancement is to control the intensity of lightwith which a particular gamma ray event is recorded in proportion to theaverage activity level which has occurred in the recent past. Forexample, apparatus is typically employed which determines the averagelevel of gamma ray activity during a time unit immediately preceding theproduction of each event pulse to be recorded. The intensity with whichthe event pulse is recorded is determined by looking at this pasthistory" of activity. For example, if the pulse occurred in an area ofhigh activity, the light intensity is increased in a nonlinear fashion,and conversely, if the pulse occurred in an area of low activity, thelight intensity is decreased in a nonlinear fashion. This mode ofoperation results in a photograph in which pulses occurring in areas ofhigh activity are blacker than normal, and in which pulses occurring inareas of low activity are whiter than normal. However, in prior artdevices, contrast enhancement has resulted in the production of anasymmetrical image and in scalloping of the type described above.

Accordingly, it is another object of the present invention to provideimproved techniques whereby contrast enhancement can be implementedwithout producing an asymmetrical image or scalloping."

SUMMARY OF THE INVENTION Applicants have been able to achieve theforegoing objects and advantages by producing an improved scintillationscanner comprising detection means for detecting a predetermined classof events for controlling the recording of individual events on arecording medium in response to the concentration of other such eventsin the class occurring in areas adjacent the individual events.

According to a principal feature of the invention, applicants providerecording means for recording the occurrence of an individual event onthe recording medium. They also provide scanning means for moving thedetection means in a first direction along a first scan path and formoving the detection means along a second scan path parallel to thefirst scan path. Means are also provided for coupling the recordingmeans and scanning means so that they are moved simultaneously.Individual event information representing an individual event in theclass occurring at an arbitrary first location along the first scan pathis stored in a storage means. Additional information is storedrepresenting other events in the class occurring before and after theindividual event in segments of the first scan path lying on both sidesof the first location at which the individual event occurred. Means arealso provided for transmitting information from the detection means tothe storage means and for also transmitting information from the storagemeans to the recording means. Control means analyze the informationstored in the storage means in order to control the recording means sothat the individual event information is recorded in response to thevalue of the stored information. In addition, means are provided formaintaining the recording means and the recording medium in a firstrelative position while the detection means is moving along the firstscan path and for maintaining the recording means and the recordingmedium in a second relative position while the detection means is movingalong the second scan path. As a result, events occurring in adjacentlocations of the first and second scan paths are recorded in adjacentlocations on the recording medium.

According to a principal feature of the method aspect of the invention,events in the class are detected along a first path segment and along asecond path segment parallel to the first path segment. Informationrepresenting the events occurring along the first path segment is storedand its value is determined. The occurrence of an event locatedsomewhere in the midsection of the first path segment is then recordedby varying a predetermined characteristic of the recording medium inproportion to the value of the stored information. Lastly, the relativepositions at which events are recorded on the recording medium in thefirst and second scan paths is altered so that the events occurring inadjacent locations of the first and second scan paths are recorded inadjacent locations of the recording medium.

By using the foregoing apparatus or method, asymmetrical variations inthe recorded image are avoided, and the scalloping" effect which hasplagued prior art techniques is completely eliminated. As a result, theexact configuration of tissue in the human body may be graphicallyrepresented with a degree of accuracy and clarity heretoforeunattainable.

DESCRIPTION OF THE DRAWINGS These and other objects, advantages andfeatures of the present invention will become apparent in connectionwith the drawings, wherein like numbers refer to like elementsthroughout, and wherein:

FIG. 1 is a perspective view of an exemplary scintillation scannerembodying a preferred form of the present invention;

FIG. 2 is a front perspective view of the control panel and cabinet ofthe scanner shown in FIG. 1;

FIG. 3 is a schematic, fragmentary, block diagram drawing of a preferredform of the present invention;

FIG. 4 is a front elevational view of the scanner shown in FIG. 1 withthe front panel thereof removed to show the scanning, detecting, andrecording assemblies thereof;

FIG. 5 is a front cross-sectional view showing a preferred form of aportion of the recording assembly shown in FIG. 4 together with apreferred form of a position control apparatus made in accordance withthe present invention;

FIG. 6 is a view taken along line 6-6 of FIG. 5;

FIG. 7 is a fragmentary view of a portion of the apparatus shown in FIG.4 taken along line 7--7 of FIG. 4;

FIG. 8 is a view taken along line 88 of FIG. 7;

FIG. 9 is a schematic diagram of a preferred form of a clock generatorand a scan stepping motor operating circuit made in accordance with thepresent invention;

FIG. 10 is a schematic diagram showing a preferred form of a storagecircuit and a gating circuit;

FIG. 11 is a schematic drawing of certain idealized voltages produced bythe gating circuit;

FIG. 12 is a schematic diagram showing a preferred form of a countingcircuit, a digital to analog generating circuit and a normalizingcircuit;

FIG. 13 is a schematic drawing of a preferred form of a background erasecircuit, a background erase threshold circuit, a contrast enhancecircuit and a contrast enhance control circuit; and

FIG. 14 is a schematic diagram of a preferred form of a light controlcircuit made in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT General Description:

Referring to FIGS. 1 3, a preferred form of the present invention wouldbasically comprise a frame assembly 12, a detecting assembly 35, arecording assembly 36, a scanning assembly 40, a storage circuit 60, anoperating circuit 70, a control system 100, and a position controlapparatus 150.

More specifically, referring to FIGS. 1 and 2, frame assembly 12comprises a base 14 which is mounted on wheels l6, l7 and 18, togetherwith another wheel mounted on the same axle as wheel 18 (not shown).Base 14 bears a pedestal 22 which supports a cabinet 24. The cabinet hasa front panel including a film cassette slot 27 and various controls andreadout devices to be described in more detail later. The cabinet alsocomprises a top panel 30 and side panels 32, 33 together with bottom andrear panels not shown so that the interior of the cabinet is completelyenclosed.

Detecting assembly 35 comprises a lead collimator (not shown) which canfocus on a small volume of gammaray emission, and a conventional gammaraysensitive crystal (not shown) which converts gamma rays passed by thecollimator into light flashes. The light flashes are amplified andconverted into electrical pulses by a photomultiplier tube (not shown).

Referring to FIG. 3, recording assembly 36 comprises a light source 37,such as a glow modulator tube, that produces a controlled beam of lightL which is passed through a variable-position planar prism 38 onto asheet of photographic film 39.

Scanning assembly 40 basically comprises a coupling boom 41 whichmechanically connects detecting assembly 35 to light source 37. Boom 41is movably supported by a carriage 42 that may be moved in pathsparallel to the Y axis by a ball screw 43 that is rotated by an indexstepping motor and operating assembly 44 and an index stepping motor44a. Movement of boom 41 in paths parallel to the X axis is achieved bya rack 45 that engages a pinion gear 46 which is mounted on a centralshaft 48. Shaft 48 is rotated in either direction by a scan steppingmotor and operating circuit 50 and a scan stepping motor 52. Steppingmotor 52 moves boom 41 a predetermined distance in response to thereceipt of a clock pulse in a well known manner.

Scanning assembly 40 is controlled by well known switching and controlcomponents in order to move detecting assembly 35 in a rectilinearscanning motion over an area of low gamma ray activity L and an area ofhigh gamma ray activity H divided by a boundary line BD. In order toachieve the rectilinear scanning motion, the scan stepping motor andoperating circuit 50 is energized so that boom 41 is moved along a scanpath parallel to the X axis, such as scan path A or D. When the boomreaches the end of a scan path, the scan stepping motor is de-energizedso that scanning motion temporarily stops. At the same time, the indexstepping motor and operating circuit is energized so that boom 41 ismoved along a path parallel to the Y axis until the next scan path isreached. For example, as the boom reaches the end of scan path A, theindex stepping motor is energized so that ball screw 43 is rotated andcarriage 42 moves the boom along the Y axis direction until detectingassembly 35 is located over scan path D. At'this time, the indexstepping motor and operating circuit is disabled, and the scan steppingmotor and operating circuit is again enabled so that the detectingassembly moves along scan path D in a direction opposite the directionit moved along scan path A. For example, if boom 41 moves in direction Falong scan path A, it is returned along scan path D in the reversedirection R. The foregoing pattern is followed until the entire area ofinterest has been scanned.

Although scan paths A and D have been shown in FIG. 3 as being separatedby a relatively wide space, it should be understood that in practice theapparatus may be arranged so that scan paths A and D are contiguous.They have only been shown in a separated manner in FIG. 3 so that therelative movement and cooperation of parts can be more clearly seen.

Still referring to FIG. 3, storage circuit 60 basically comprises adigital shift register 62 and an identical shift register 64 that arejoined by a common conductor 66. Bits of data may be shifted from shiftregister 62 to shift register 64 through conductor 66, and data may beshifted out of shift register 64 through a conductor 68.

Operating circuit 70 basically comprises a time interval selector72which is connected to a clock generator 76 through a conductor 74.Selector 72 controls the frequency of pulses produced by clock generator76 on a conductor 78 through a control located on panel 26 7 (FIG. 2).By varying the frequency of pulses produced by clock generator 76, timeinterval selector 72 controls the speed with which the scan steppingmotor moves boom 41 along paths A and D. This is achieved because scanstepping motor 52 moves boom 41 a predetermined distance in response tothe receipt of every 20th clock pulse produced by the clock generator.In addition to operating stepping motor 52, the clock generator alsoprovides pulses to shift registers 62, 64 and to a gating circuit 86that operates the shift registers.

The operating circuit also comprises a pulse height analyzer 80 whichreceives electrical pulses corre sponding to gamma ray events from thephotomultiplier tube in detecting assembly 35 over a conductor 82. In awell known manner, the pulse height analyzer produces output pulses onan output conductor 84 only in response to electrical pulses havingamplitudes within a predetermined range of values. Such pulse heightanalyzers are well known in the art, and are generally used inscintillation scanners in order to prevent the counting of gamma rayshaving energy levels outside a known energy spectrum. The output ofpulse height analyzer 80 is connected to gating circuit 86 whichincludes buffer circuits that temporarily store the output pulse untilthey can be shifted into the shift registers in synchronism with theclock pulses produced by the clock generator. In this manner,information bits representing gamma ray events can be moved through theshift registers in a manner which coincides with the scanning positionsof detecting assembly 35. In other words, gating circuit 86 makes surethat information is clocked through shift registers 62 and 64 at a ratewhich is directly proportional to the distance covered by detectingassembly 35. As a result of this unique apparatus, the amount of gammaray activity along any particular segment of scan path A or D may beeasily determined. After the electrical pulses have been synchronizedwith the clock generator and gating circuit 86, they are transmitted toshift register 62 over an output conductor 88.

Control system 100 basically comprises an accounting circuit 102 thatreceives input pulses from an output of gating circuit 86 over a cable104 and from the output of shift register 64 over conductor 68. Theaccounting circuit produces an output corresponding to a binary numberwhich represents the total number of bits of information stored in shiftregisters 62 and 64. This number is transmitted over a series ofconductors shown schematically as conductor 106 to a digital to analoggenerating circuit 110. Generating circuit 110 produces a DC analogvoltage on an output conductor 112 having a magnitude which isproportional to the value of the number produced by the accountingcircuit. The signal appearing on conductor 112 is attenuated in anormalizing circuit 114 which is controlled by time interval selector 72through a conductor 116. Basically, normalizing circuit 114 attenuatesthe analog signal produced by generating circuit 110 so that it normallycovers a range of about volts irrespective of the pulse frequency ofclock generator 76. The normalized voltage is transmitted over an outputconductor 118 to a background erase circuit 120 which is controlledthrough a conductor 122 by a background erase control 124 located onfront panel 26 (FIG. 2). The background erase circuit 120 compares thenormalized DC voltage appearing on conductor 118 with a thresholdcontrol voltage appearing on conductor 122. If the normalized voltage isgreater than the threshold voltage, thereby indicating a significantamount of gamma ray activity in the area being scanned, background erasecircuit 120 produces an output signal on an output conductor 126 that istransmitted to a contrast enhance circuit 128. At the same time, anoutputsignal is generated over a conductor 140 that enables an eventinformation pulse transmitted over a conductor 142 from between theshift registers to pass through an AND gate 143 and a conductor 145 toenergize a light control circuit 136. If the normalized voltageappearing on conductor 118 is less than the threshold voltage appearingon conductor 122, no output pulse is generated on conductor 140, so thatno light pulse is produced. Background erase control circuit 124 iscapable of producing a range of threshold voltage signals that cansubstantially vary the degree of gamma ray activity required in order toenergize light control circuit 136.

Contrast enhance circuit 128 is connected to a contrast enhance control132 through conductors 130, 131. Contrast enhance control 132 contains agroup of nonlinear elements which control a nonlinear amplifier thatoperates on the normalized voltage received from background erasecircuit 120 to prodice a control voltage. The control voltage istransmitted over a conductor 134 that operates light control circuit 136in a nonlinear manner. As a result, the intensity of light pulsesproduced by light source 37 is regulated so that light pulses occurringin an area of relatively high gamma ray activity make a relatively darkimage on film 39 and pulses occurring in an area of relatively low gammaray activity produce a relatively light image on film 39.

As shown in FIG. 3, light control circuit 136 transmits a control signalover a conductor 144 in order to vary the intensity of the lightproduced by light source 37. As previously explained, if the properoutput voltage is not produced on conductor 140, no light pulse at allis produced by source 37. Light beam L produced by source 37 istransmitted through a flat prism 38 which can be pivoted into twodifferent positions by position control apparatus 150 which operates acontrol rod 152 connected to the prism frame. In the first position,prism 38 refracts the light beam so that it moves one-quarter centimeterin the R direction, and in the second position, prism 38 refracts thelight beam so that it moves one-quarter centimeter in the F direction.As will be explained in more detail later, the prism must be pivoted toits different positions on successive scan paths so that gamma raysoccurring in adjacent locations of paths A and D are recorded inadjacent locations on paths A and d of film 39.

The operation of the apparatus referred to in the above generaldescription will now be described assuming that scan stepping motor andoperating circuit 50 is moving boom 41 along scan path A in direction Fso that detecting assembly 35 receives radiation from point S on path A.As boom 41 is moved in direction F, gamma rays are detected andconverted to electrical pulses by detecting assembly 35. The electricalpulses transmitted by pulse height analyzer are clocked by gatingcircuit 86 and are shifted into shift register 62 where they are storedas individual information bits each representing a gamma ray event.Assuming the gamma ray activity level in area L is below the thresholdlevel established by background erase control 124, no light pulses areproduced by light source 37, and no image is produced on film 39.However, bits of information representing gamma ray events are shiftedone position in shift register 62 upon the receipt of each clock pulseover conductor 78. At the same time clock generator 76 is shifting gammaray event information bits through shift register 62, it is alsoproducing pulses which are divided by 20 and are used to drive scanstepping motor 52. As previously explained, stepping motor 52 isdesigned so that it moves boom 41 a predetermined incremental distancein response to the receipt of each clock pulse. As a result, it ispossible to determine the level of gamma ray activity occurring on anyparticular segment of scan line A by merely determining the number ofinformation bits clocked through the shift register in response to theclock pulses which caused the stepping motor to move through that scanline segment. In the present embodiment, scan stepping motor andoperating circuit 50 is designed so that the clock pulses required tomove boom 41 one-quarter centimeter along scan line A are alsosufficient in number to move a bit of information from input conductor88 to output conductor 66 of shift register 62. For example, assumingpath segment B is one-quarter centimeter in length, the bits ofinformation corresponding to gamma ray events occurring in segment Bbegin to be shifted into shift register A as detecting assembly 35passes over borderline BD.

As detecting assembly 35 is moved over line BD in the F direction, gammaray events occurring just inside area H are detected. It is assumed thatin area H there is a high gamma ray activity level in excess of thethreshold level established by background erase control 124. Accordingto a novel feature of the invention, the gamma ray event occurring atborderline BD does not immediately produce a light pulse which is usedto expose film 39. Instead, the information bits from scan path segmentB which are stored in shift register 62 are shifted bit-by-bit intoshift register 64, and information bits representing gamma ray eventsoccurring along scan path segment C are shifted into shift register 62in place of the previous data. Path segment C is also onequartercentimeter in length. By the time detecting assembly 35 is moved to theposition shown in solid lines in FIG. 3 (i.e., the position over pointT), shift register 64 contains information bits from line segment B andshift register 62 contains information bits from path segment C. At thispoint in time, the information bit representing the gamma ray eventoccurring at borderline ED is transmitted in the form of a square-wavevoltage pulse over conductors 66 and 142 to AND gate 143.

As detecting assembly 35 has been moved over path segment C, accountingcircuit 102 has determined the total bits of information stored in shiftregisters 62 and 64. This information continuously is converted to ananalog DC potential by circuit 110 and is normalized by circuit 114 inthe manner previously described. The normalized voltage is compared withthe threshold potential in the background erase circuit 120. Since areaH is an area of high gamma ray activity, the accumulation of informationbits in shift register 62 from line segment C increases the averagenumber of information bits stored in both shift registers. At thispoint, it will be assumed that the average level of gamma ray activityin scan path segments B and C combined exceeds the threshold levelestablished by background erase control 124 just as detecting assembly35 passes over point T and light source 37 passes over point Tl. As aresult, an output signal is produced on conductor 140 which switches ANDgate 143 to its 1 state, thereby causing light control circuit 136 toallow the production of a beam of light L by light source 37. Light beamL is refracted one-quarter centimeter in the R direction by prism 38 sothat it is projected onto line BDl instead of point Tl. I

It should be noted that a gamma ray event occurring at line BD has itsrecording controlled by gamma ray events occurring both before and afterline BD in scan line segments B and C. This is an important feature,since it enables the system to accurately evaluate the extent to whichthe gamma ray event occurred in an area of low gamma ray activity, highgamma ray activity, or on the borderline of such areas.

As detecting assembly 35 continues to be moved in the F direction,additional bits of information resulting from gamma ray events occurringin area H are shifted into shift registers 62 and 64 so that thethreshold level established by background erase control 124 iscontinually exceeded. As shown in FIG. 3, pulses of light recordinggamma ray events in area H continue to expose film 39.

it should be noted that the system may begin to expose film 39 in anarea just before line 8D] or just after line BDl depending on the exactsetting of background erase control 124. However, as will be describedlater, if the system begins to produce pulses before line BDI whilescanning in the F direction, it produces pulses after line BD1 whilescanning in the R direction, so that gamma ray events occurring inadjacent locations on paths A and D are recorded in adjacent locationson film 39.

When detecting assembly 35 is moved to the extreme end position in the Fdirection along scan path A, a

switch is thrown so that scan stepping motor and operover the scan pathD. At this time, index stepping motor and operating circuit 44 isde-energized and scan stepping motor and operating circuit 50 isre-energized in the reverse direction, so that detecting assembly 35 andlight source 37 begin to travel in the R direction along scan path D.During this period of time, prism 38 is switched to its second positionso that light beam L is refracted one-quarter centimeter in the Fdirection. As detecting assembly 35 passes over point U of scan path Din the R direction, information bits representing gamma ray eventsoccurring along path segment E begin to enter shift register 62. Asdetecting assembly 35 passes over line BD of scan path D (i.e., theposition shown in phantom at P information bits representing gamma rayevents occurring in path segment E begin to enter shift register 64, andinformation bits representing gamma ray events occurring in path segmentF begin to enter shift register 62. As previously noted, the recordingof a gamma ray event occurring at line ED is delayed until the detectingassembly travels another quarter centimeter, i.e., until it attains theposition shown in phantom at P in which detecting assembly 35 ispositioned over point V and light source 37 is positioned over point V1.At this time, the average number of information bits stored in shiftregisters 62 and 64 is still higher than the threshold level establishedby background erase control 124. As a result, background erase circuitproduces a signal on output conductor 140, and AND gate 143 is switchedto its 1 state, thereby enabling light source 37 to produce a lightpulse which is focused on film 39. It will be noted that light beam Lpasses through prism 38 in its second position, so that the beam isrefracted one-quarter centimeter in the F direction. As a result, it isfocused adjacent line BDl instead of position V1;

As detecting assembly 35 is moved beyond position V, the average numberof information bits stored in shift registers 62 and 64 combined fallsbelow the threshold level set by background erase control circuit 124 sothat gamma rayevents occurring just beyond the BD line in the L arearesult in no light pulses.

Of course, it will be clear to those skilled in the art that lightpulses will not be produced exactly at line BDl in the manner previouslydescribed if background erase control 124 is set to a different level.If the control is adjusted to a lower threshold level, light pulses willbegin to be produced in segment BDl of scan path A rather than at lineBD1. However, the reverse mode of operation will occur on scan path D sothat light I pulses will not be produced until line segment Fl has beenpenetratedto a point adjacent the corresponding images appearing in linesegment B1 of scan path A. As a result, borderline BD will berepresented by a similar line on film 39 parallel to and slightlydisplaced from .higher threshold leveli the reverse process will takeplace so that border line BD will be represented on film 39 by a lineparallel to line BDl which passes through line segments Cl and E1.

The foregoing description has been given assuming that the contrastenhance circuit 128 was inoperative. However, those skilled in the artwill recognize that a similar mode of operation can be achieved in whichscalloping is eliminated when contrast enhance circuit 128 is energized.The operation of this circuit results in an image of increased opacityin film path segments C1 and E1 in which no scalloping effect appears.

DETAILED DESCRIPTION Recording Assembly 36 Referring to FIG. 4,recording assembly 36 comprises a light-tight film chamber 160 formedinside cabinet 24. Boom 41 extends into chamber 160 through a slotlocated in a wall 162 in the central portion of the cabinet. When thescanner is in use, a film cassette 164 is entered through access slot 27so that film sheet 39 is positioned directly below a light sourceassembly 170 which is rigidly mounted to boom 41.

Referring to FIGS. and 6, light source assembly 170 comprises side walls172 and 173, a cover 174, a top plate 175, and a bottom plate 176 thatis drilled with a hole to allow light to focus on film sheet 39.

As shown in FIG. 5, light source 37 comprises a glow modulator tube thatis held in position by a shield 177 and by a locator spring 178 that aremounted on a bracket 179.

The light beam produced by tube 137 is transmitted in a downwarddirection through a rotatable aperture disc 180 that is drilled with anumber of aperture holes which vary in size. Disc 180 is centrallymounted on a shaft 181 that is journaled in an upper bearing 182 and alower bearing 183 located in a horizontal support plate 184. Theaperture disc may be rotated from a remote location by a gear andratched device 185.

The light beam generated by tube 37 is transmitted downward through afocusing lens 186 and a prism assembly 190 shown in more detail in FIG.6.

Prism assembly 190 comprises a housing 191 having fourvertically-standing walls that support flat planar prism 38. The prismis held in a frame 192 that is connected to shafts 193 and 194. Theshafts are rotatably mounted through housing 191. Shaft 194 is connectedto a bracket 195a which is, in turn, rotatably mounted in an arm 195that is biased in the position shown by a return spring 196. Arm 195 canbe moved to an energized position by a solenoid 197 that operates thearm through a drive rod 198. The solenoid is energized while one path isscanned and is de-energized while the next subsequent path is scanned sothat the light beam transmitted by tube 37 is refracted from its normalposition by a distance of one-quarter centimeter in one of twodirections depending on the position of the prism. After the light isrefracted by prism 38, it is projected through a hold in bottom plate176 onto film sheet 39 as shown in FIG. 4.

SCANNING ASSEMBLY 40 Referring to FIGS. 4, 7, and 8, index steppingmotor and operating circuit 44 comprises vertical support walls 200 and201 which support ball screw 43 through a bearing 203 mounted in wall200 and a like bearing mounted in wall 201 (not shown). Ball screw 43 ismechanically attached to a pulley 204 that is driven by a pulley 205through a belt 207. Pulley 205 is connected to the shaft of indexstepping motor 44a.

Referring to FIG. 8, scanning stepping motor and operating circuit 50comprises a double-hub pulley 212 which drives pinion gear 46 from itsinner hub through a belt 213. As previously described, pinion gear 46 isengaged with a rack 45 that is mechanically connected to boom 41. Theouter hub of pulley 212 is driven by another pulley 216 through a belt215. Pulley 216, in

turn, is driven by scan stepping motor 52 through an appropriate driveassembly 220.

Referring to FIG. 9, scan stepping motor 52 is advanced by a scanstepping motor operating circuit 222. This circuit comprises a forwardswitch 224 that is engaged when boom 41 reaches one end of a scan pathand a reverse switch 226 that is engaged when boom 41 reaches theopposite end ofa scan path. These switches provide command signals to aconventional Gray code generator 228 that generates appropriate pulsesfor stepping the scan stepping motor 52. The frequency of operation ofthe Gray code generator is determined by the clock pulses received onconductor 78 from clock generator 76. The clock pulses are passedthrough a decade counter 230 which divides the frequency of the pulsesby 10. The output of the decade counter is connected to the clock inputof a bistable flipflop 232 that divides the pulse frequency by 2. Theoutput of the flipflop is connected through an amplifier 234 to oneinput of the Gray code generator. As a result of this arrangement, thefrequency of the clock pulses received on conductor 78 is divided by 20in order to operate scan stepping motor 52. As previously mentioned,scan stepping motor 52 rotates through a precise, predetermined arc inresponse to the receipt of each clock pulse by generator 228. The outputof the Gray code generator is connected through a conventional steppingmotor drive 236 that provides sufficient current to drive the motor.

OPERATING CIRCUIT Referring to FIG. 9, operating circuit comprises aunijunction oscillating circuit 250 that is controlled by apotentiometer 240 comprising time interval switch 72. Circuit 250 alsocomprises capacitors 251254, resistors 255257, an inductor coil 259,diodes 261, 262, a junction transistor 263, a unijunction transistor264, an amplifier 265, and a NAND gate 267, all connected as shown. Theunijunction oscillator circuit further comprises a one-shotmultivibrator 270 consisting of capacitors 272,273, resistors 275, 276,a diode 278, and NOR gates 280, 281. As is well known to those skilledin the art, a unijunction transistor 264 produces pulses at a frequencybasically determined by the value of potentiometer 240 and the values ofcapacitors 251, 252, and 254 connected to the gate terminal thereof.One-shot multivibrator 270 produces well-defined, square-wave pulses inresponse to the signals generated by unijunction transistor 264. Circuit250 also comprises an inverting NOR gate 282 that is connected to outputconductor 78.

Referring to FIG. 10, gating circuit 86 comprises a high-speed clockcircuit 290 consisting of a crystalcontrolled oscillator 292 that hasits output frequency divided by flipflop circuits 294 and 295.Oscillator 292 is used as a high-speed clock to introduce 0 state pulsesinto shift registers 62 and 64 to clear the registers at the end ofascan line. Additional pulses used to operate the shift registers andgating circuit are supplied on conductor 78 from clock generator 76. Aselecting network consisting of NAND gates 297-299 provides the logicwhich determines whether the circuitry is operated by crystal-controlledoscillator 292 or the clock pulses transmitted on conductor 78.Conductor 293 is switched to its 0 state whenever boom 41 is travelingalong a scan path. The output of NAND gate 297 is transmitted over aconductor 296 in order to clock the binary counter shown in FIG. 12hereafter.

Gating circuit 86 also comprises a phase 1 phase 2 circuit 300*thatgenerates the clocking pulses shown schematically in FIG. 11. Morespecifically, circuit 300 comprises a bistable flipflop 302, NAND gates304,

305, inverting amplifiers 307, 308, and resistors 309,

310. As shown in FIG. 10,.the phase 1 signal shown in FIG. 11 isgenerated at the output of NAND gate 304 and the phase 2 signal shown inFIG. 11 is generated at the output of NAND gate 305.

Gating circuit 86 also comprises a level-shifting circuit 312 consistingof resistors 314-319 and transistors 321-324. Another level shiftingcircuit 326 consists of resistors 328-331 and transistors 333, 334, allconnected as shown.

Gating circuit 86 also comprises an information clocking circuit 336consisting of buffer flipflops 338, 342, and a reset flipflop 340. Theclocking circuit also consists of inverting amplifiers 344, 346, atransistor 347, resistors 348 350, and a NAND gate 352.

The information clocking circuit synchronizes the input of data from thepulse height analyzer over conductor 84 into shift registers 62 and 64in the following manner. A pulse on conductor 84 switches the Q outputof flipflop 338 to its '1 state, thereby enabling NAND gate 352 toswitch to its state as soon as an inverted phase 2 clock pulse isreceived from NAND gate 305. As soon as NAND gate 352 is switched to its0 state, flipflop 342 is reset so that its Q output is switched to its 0state-and its 6 output is switched to its 1 state. Flipflop 342 remainswith its 6 output in the 1 state until the next phase 1 pulse isreceived from the emitter of transistor 323. At this time, the 1 stateof the 0 output of flipflop 342 is transmitted through level shiftingcircuit 326 and conductor 88 ahd is shifted into shift register 62. Asthe phase 1 pulse terminates, the 6 output of flipflop 342 returns toits 0 state and the Q output returns to its 1 state. At this time, resetflipflop 340 is clocked so that flipflop 338 is reset and is madeavailable to accept an additional event pulse from conductor 84. Theresetting of flipflop 338, in turn, resets flipflop 340.

As'previously described, the foregoing operation enables data to beshifted through the shift registers in synchronism with the positionalmovement of the scan stepping motor. Of course, the bits of informationstored in the shift registers are shifted in synchronism with the clockpulses appearing on conductor 78 even though no new data is entered onconductor 84. As previously described, information bits are shifted fromregister 62 to 64 through a conductor 66. In addition, the informationbits are also conducted through a NAND gate 354 to conductor 142 whichis connected to AND gate 143 (FIG. 3).

CONTROL SYSTEM Referring to FIGS. 10 and 11, accounting circuit 102comprises a gate circuit 356 consisting of NAND gates 358-361, invertingamplifiers 362-364 and output conductors 365 and 366. Accounting circuit102 also comprises binary counters 370-372 which count up if conductor366 is in its 1 state and count down if conductor 366 is in its 0 state.In addition, counters 370-372 are enabled to count if conductor 365 isin its 1 state and are disabled from counting if conductor 365 is in its0 state.

Gating circuit 356 is arranged so that binary counters 370-372 count upif a bit of information is shifted into the shift registers, but no bitof information is shifted out (i.e., the l in-0 out case), and isarranged so that the counters count down if no bit of information isshifted into the shift registers, but a bit of information is shiftedout (i.e., the 0 in-l out case). In addition, circuit 356 is arranged sothat the binary counters are disabled from counting if a bit ofinformation is shifted into the shift registers at the-same time a bitof information is shifted out (i.e., the 1 in-l out case), and thebinary counters are also disabled if no bit of information is shiftedinto or out of the shift registers (i.e., the 0 in-0 out case). In orderto achieve the foregoing results, NAND gate 361 is switched to its 0state for the 1 in-0 out case and is switched to its 1 state for the 0in-l out case. The binary counters are enabled or disabled through theoperation of NAND gates 358-360. Basically, NAND gate 360 is alwaysswitched to its 1 state except when the input conductor from NAND gates358 and 359 is switched to its 0 state. This occurs in the 1 in-l outcase when NAND gate 358 is switched to its 0 state, and in the 0 in-0out case when NAND gate 359 is switched to its 0 state. For these twocases, NAND gate 360 is switched to its 1 state so that the counters aredisabled through inverting amplifier 363.

Referring to FIG. 12, digital to analog converter comprises invertingamplifiers 374-383 and associated resistors 384-393, respectively, allconnected as shown. The values of resistors 384-393 are varied so thatthe resulting DC voltage on the outputs of the resistors has a magnitudewhich corresponds to the magnitude of the binary number stored in thebinary counters 370-372. The output of the resistors is connected to anamplifying circuit 395 comprising resistors 397-404, capacitors 406,407, and a conventional amplifier 409. The output of amplifying circuit395 is transmitted over a conductor 112 to normalizing circuit 114 whichcomprises an amplifier 411, a resistor 412, and an adjustablepotentiometer 413 connected as shown. Potentiometer 413 is ganged withthe potentiometer 240 in the time interval switch circuit.Potentiometers 240 and 413 are arranged so that the voltage produced onconductor 118 ranges from about 0 to 5 volts irrespective of thefrequency of clock generator 76 or the rate of movement of boom 41.

Referring to FIG. 13, background erace circuit 114 comprises resistors420-447, capacitors 450-468, diodes 470-475, and amplifiers 480-484. Aspreviously described background erase circuit 1 14 is controlled by athreshold voltage which is transmitted over conductor 122 from abackground erase control 124. As shown in FIG. 13, control 124 comprisesa potentiometer 486. The threshold voltage transmitted over conductor122 is current summed with the normalized enhancement voltagetransmitted over conductor 118 in a summing circuit 488 comprisingresistors 439 and 440. The voltage resulting from the comparator circuitis expanded by amplifier 483, the gain of which is controlled by avariable potentiometer 442 in the feedback circuit thereof. After againbeing amplified by amplifier 484, the resulting voltage is transmittedto the contrast enhance circuit 128 over conductor 126.

Contrast enhance circuit 128 comprises resistors 490-498, capacitors500-503, diodes 506-509, transistors 512,513, and an amplifier 515.

As previously described, the contrast enhance circuit is controlled by acontrast enhance control 132 which comprises resistors 520-532 anddiodes 534-541. Various combinations of the resistors and diodes ofcontrast enhance control 132 may be placed across conductors 130, 131,by switching means not shown. As a result, the degree of contrastenhancement may be controlled by the operator. It should be noted thatthe contrast enhance control circuitry is connected in the feedback loopof amplifier 515.

The contrast enhance control voltage is generated on conductor 134through which it is transmitted to light control circuit 136.

Referring to FIG. 14, light control circuit 136 comprises resistors546-554, capacitors 556,557, a diode 558, and transistors 560-563.

If the normalized voltage transmitted on conductor 118 exceeds thethreshold level established by control 124 (FIG. 3), AND gate 143 isenabled by the next pulse produced by shift register 62 so that asquarewave voltage pulse is transmitted over conductor 145 to transistor563 (FIG. 14). As a result, transistor 563 is switched on and transistor562 is switched off so that the contrast enhance voltage transmittedover conductor 134 is conducted to transistor 561. Transistor 561, inturn, controls the amount of current flowing through transistor 560which acts as a current sink for regulating the current flow throughglow tube 37. After the tube begins to conduct, the intensity ofillumination generated by the tube is controlled by the voltageappearing on conductor 134 until the pulse controlling AND gate 143terminates. At this point in time, transistor 563 is switched off andtransistor 562 is switched on so that the voltage available at conductor134 is shunted to ground. Thereafter, transistor 560 prevents theconduction of additional current through glow tube 37. As a result, thetube ceases to produce light until the next pulse is received from ANDgate 143.

Those skilled in the art will recognize that the preferred embodimentdescribed herein is merely exemplary of the preferred practice of theinvention, and that modifications and alterations may be made withoutdeparting from the spirit and scope of the invention.

We claim:

1. in a scintillation scanner comprising detection means for detecting apredetermined class of events, improved apparatus for controlling therecording of individual events on a recording medium in response to theconcentration of other such events in the class occurring in the areasadjacent the individual events, comprising in combination:

recording means for recording the occurrence of an individual event inthe class on the recording me-- dium;

scanning means for moving the detection means in a first direction alonga first scan path and for moving the detection means along a second scanpath parallel to the first scan path;

coupling means for coupling the detection means to the recording meanswhereby the detection means and the recording means are movedsimultaneously;

storage means for storing individual event information representing anindividual event in the class occurring at an arbitrary first locationalong the first scan path and for storing information representing otherevents in the class occurring before and after the individual event insegments of the first scan path lying on both sides of the firstlocation;

operating means for transmitting information representing events in theclass from the detection means to the storage means and for operatingthe storage means;

transmitting means for transmitting the individual event informationfrom the storage means to the recording means; control means foranalyzing the information stored in the storage means and forcontrolling the recording means so that the individual event informationis recorded in response to the value of the stored information; andmeans for maintaining the recording means and the recording medium in afirst relative position while the detection means is moving along thefirst scan path and for maintaining the recording means and therecording medium in a second relative position while the detection meansis moving along the second scan path, whereby events occurring inadjacent locations of the first and second scan paths are recorded inadjacent locations on the recording medium. 2. Apparatus, as claimed inclaim 1 wherein the recording means comprises a beam of light and meansfor generating the beam of light.

3. Apparatus, as claimed in claim 2, wherein the means for generatingcomprises a glow modulator tube.

4. Apparatus as claimed in claim 1, wherein the scanning means comprisesstepping motor means adapted to receive a clock pulse for moving thedetection means a predetermined distance in response to each clockpulse.

5. Apparatus, as claimed in claim 1, wherein the coupling meanscomprises a boom on which the detection means and the recording meansare mounted.

6. Apparatus, as claimed in claim 1, wherein the storage meanscomprises:

first storage means for storing said individual event information andfor storing information representing events occurring in a first segmentof the first scan path extending from the first location in the firstdirection; second storage means for storing information representingevents occurring in a second segment of the first scan path extendingfrom the first location in a second direction opposite the firstdirection; and

means for transferring information between the first and second storagemeans.

7. Apparatus, as claimed in claim 6, wherein the first storage meanscomprises a first shift register means for storing digital informationand wherein the second storage means comprises a second shift registermeans for storing digital information.

8. Apparatus, as claimed in claim 1, wherein the operating meanscomprises means for retaining the individual event information in thestorage means until the information representing said other eventsoccurring before and after the individual event has been stored in thestorage means.

9. Apparatus, as claimed in claim 7, wherein the scanning meanscomprises stepping motor means adapted to receive a clock pulse formoving the detection means a predetermined distance in response to

1. In a scintillation scanner comprising detection means for detecting apredetermined class of events, improved apparatus for controlling therecording of individual events on a recording medium in response to theconcentration of other such events in the class occurring in the areasadjacent the individual events, comprising in combination: recordingmeans for recording the occurrence of an individual event in the classon the recording medium; scanning means for moving the detection meansin a first direction along a first scan path and for moving thedetection means along a second scan path parallel to the first scanpath; coupling means for coupling the detection means to the recordingmeans whereby the detection means and the recording means are movedsimultaneously; storage means for storing individual event informationrepresenting an individual event in the class occurring at an arbitraryfirst location along the first scan path and for storing informationrepresenting other events in the class occurring before and after theindividual event in segments of the first scan path lying on both sidesof the first location; operating means for transmitting informationrepresenting events in the class from the detection means to the storagemeans and for operating the storage means; transmitting means fortransmitting the individual event information from the storage means tothe recording means; control means for analyzing the information storedin the storage means and for controlling the recording means so that theindividual event information is recorded in response to the value of thestored information; and means for maintaining the recording means andthe recording medium in a first relative position while the detectionmeans is moving along the first scan path and for maintaining therecording means and the recording medium in a second relative positionwhile the detection means is moving along the second scan path, wherebyevents occurring in adjacent locations of the first and second scanpaths are recorded in adjacent locations on the recording medium. 2.Apparatus, as claimed in claim 1 wherein the recording means comprises abeam of light and means for generating the beam of light.
 3. Apparatus,as claimed in claim 2, wherein the means for generating comprises a glowmodulator tube.
 4. Apparatus as claimed in claim 1, wherein the scanningmeans comprises stepping motor means adapted to receive a clock pulsefor moving the detection means a predetermined distance in response toeach clock pulse.
 5. Apparatus, as claimed in claim 1, wherein thecoupling means comprises a boom on which the detection means and therecording means are mounted.
 6. Apparatus, as claimed in claim 1,wherein the storage means comprises: first storage meanS for storingsaid individual event information and for storing informationrepresenting events occurring in a first segment of the first scan pathextending from the first location in the first direction; second storagemeans for storing information representing events occurring in a secondsegment of the first scan path extending from the first location in asecond direction opposite the first direction; and means fortransferring information between the first and second storage means. 7.Apparatus, as claimed in claim 6, wherein the first storage meanscomprises a first shift register means for storing digital informationand wherein the second storage means comprises a second shift registermeans for storing digital information.
 8. Apparatus, as claimed in claim1, wherein the operating means comprises means for retaining theindividual event information in the storage means until the informationrepresenting said other events occurring before and after the individualevent has been stored in the storage means.
 9. Apparatus, as claimed inclaim 7, wherein the scanning means comprises stepping motor meansadapted to receive a clock pulse for moving the detection means apredetermined distance in response to each clock pulse and wherein theoperating means comprises: clock means for generating clock pulses at apredetermined rate; means for transmitting the clock pulses to thestepping motor means; buffer means for temporarily storing informationcorresponding to one of said events and for transferring the informationinto one of the shift register means in response to a clock pulse; andmeans for transmitting the clock pulses to the first and second shiftregister means, whereby information is advanced through the first andsecond shift register means in response to the clock pulses. 10.Apparatus, as claimed in claim 9, wherein the transmitting means isconnected between the first shift register means and the second shiftregister means, whereby information is transmitted by the transmittingmeans to the recording means as it is shifted from one of the shiftregister means to the other shift register means.
 11. Apparatus, asclaimed in claim 1, wherein the control means comprises: accountingmeans for analyzing the information stored in the storage means;generating means for generating a control signal having a valuecorresponding to the value of the information analyzed by the accountingmeans; and driving means for controlling the recording means in anon-linear manner in proportion to the value of the control signal,whereby the contrast with which the event information is recorded isenhanced.
 12. Apparatus, as claimed in claim 1, wherein the controlmeans comprises means for disabling the recording means if the value ofthe stored information is below a preset threshold level.
 13. Apparatus,as claimed in claim 11, wherein the accounting means comprises means fordetermining the total number of bits of information held in the storagemeans and wherein the generating means comprises a digital-to-analogconverter.
 14. Apparatus, as claimed in claim 13, wherein the means fordetermining comprises: a counter capable of counting up and countingdown; and gate means comprising first means for enabling the counter tocount up when the value of information entering the storage means isgreater than the value of information leaving the storage means, secondmeans for enabling the counter to count down when the value ofinformation entering the storage means is less than the value ofinformation leaving the storage means, and the third means for disablingthe counter when the value of information entering the storage means isequal to the value of information leaving the storage means. 15.Apparatus, as claimed in claim 2, wherein the means for maintainingcomprises means for deflecting the beam of light.
 16. Apparatus, asclaimed in claim 15, wherein the means for deflecting comprises: Aprism; means for positioning the prism in the beam of light; and meansfor maintaining the prism in a first position relative to the beam oflight when the detection means is moving along the first scan path andfor maintaining the prism in a second position relative to the beam oflight when the detection means is moving along the second scan path. 17.A method of controlling the manner in which a predetermined class ofevents is recorded in a scintillation scanner on a recording medium by arecording means having a variable output value capable of altering apredetermined characteristic of the recording medium, said methodcomprising the steps of: detecting events in the class occurring along afirst path segment having a predetermined length and having first andsecond end points; detecting events along a second path segment parallelto the first path segment; storing information representing said eventsoccurring along the first path segment; determining the value of thestored information; recording the occurrence of an event located in thefirst path segment at a location other than at the first or second endpoints by varying the predetermined characteristic of the recordingmedium in proportion to the value of the stored information; andaltering the relative positions at which events are recorded on therecording medium in the first scan path as compared with the second scanpath, whereby events occurring in adjacent locations of the first andsecond scan paths are recorded in adjacent locations of the recordingmedium.
 18. A method, as claimed in claim 17, wherein the step ofstoring comprises the steps of: converting the occurrence of an eventinto stored information during spaced intervals of time occurring at aconstant rate, and shifting the storage location of the information atsaid constant rate.
 19. A method, as claimed in claim 17, wherein thestep of determining the value comprises the step of determining thetotal number of bits of information stored.
 20. A method, as claimed inclaim 17, wherein the steps of recording comprises the steps of:generating a beam of radiant energy; transmitting the beam toward therecording medium; regulating the intensity of the beam in proportion tothe value of the stored information.
 21. A method, as claimed in claim20, wherein the step of altering comprises the step of directing thebeam along a first path when events are being detected in the first pathsegment and directing the beam along a second path when events are beingdetected in the second path.